Flood Risk Assessment in Elmbridge

A development in Elmbridge has to satisfy two authorities on two fronts — flood risk and drainage — and knowing which body wants what is the difference between a clean submission and a stalled one. Elmbridge Borough Council determines your planning application against its adopted Local Plan, while Surrey County Council, as the Lead Local Flood Authority, sets the surface water drainage and SuDS requirements. Unda prepares the flood risk assessments and drainage strategies that satisfy both, for planning, property purchase, mortgage and insurance, and BREEAM — alongside sequential and exception tests.

Elmbridge judges flood risk against Core Strategy Policy CS26, and Surrey County Council sets the SuDS discharge rules — so on a site here you are answering both a flood-risk test and a drainage test, and much of the borough sits in Flood Zones 2 and 3 on the tidal-free Thames and the lower Mole, Ember and Rythe.

If you want a scheme in the borough reviewed, contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

• Planning authority. Elmbridge Borough Council determines the application and holds the adopted flood risk policies your report must meet.
• Lead Local Flood Authority. Surrey County Council is consulted on surface water drainage and sets the SuDS and discharge requirements for the site.
• Flood policy backbone. Core Strategy Policy CS26 (Flooding, 2011), Development Management Plan Policies DM13 (Riverside development) and DM5 (Pollution, carrying the SuDS requirement), and the Flood Risk SPD (adopted 3 May 2016).
• Drainage expectation. SuDS down the hierarchy, discharge normally limited to the greenfield runoff rate, and the 1-in-30 and 1-in-100 (plus climate change) storms managed.
• Change ahead. From 1 April 2027 Elmbridge becomes part of the new East Surrey unitary council, though the flood risk and drainage tests themselves do not change.

Who decides flood risk and drainage on an Elmbridge application?

Three bodies shape the flood risk and drainage side of an Elmbridge application. Elmbridge Borough Council is the decision-maker and applies planning policy. Surrey County Council, as Lead Local Flood Authority, advises on surface water and SuDS. The Environment Agency advises on the main rivers — the Thames, Mole, Ember and Rythe. Getting the right report to the right body is half the battle.

The split between council and county is the single most common source of confusion here. Our consultants deal with the Environment Agency and Surrey County Council directly, so the drainage strategy answers the LLFA's questions before an LLFA objection is raised. If you want to understand how these roles fit together nationally, our explainer on the role of Lead Local Flood Authorities and on how local planning authorities handle flood and SuDS sets it out in full.

Does an Elmbridge development need a flood risk assessment or a drainage strategy?

A site-specific flood risk assessment is required for a development in Flood Zone 2 or 3, for any site of one hectare or more even in Flood Zone 1, where the council's mapping shows surface water risk, or where a more vulnerable use is proposed. A drainage strategy is a separate report and is triggered by 10 or more dwellings, a hectare or more, or 1,000m² of floorspace — and often on smaller sites in flood-risk or surface-water areas. The two frequently travel together, but they answer different questions.

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. A clean assessment that shows a site is not actually within an area at risk can sometimes remove the need for the sequential test at application stage. Missing or inadequate flood and drainage information is one of the most common reasons a scheme is held up — see our note on why a planning application is invalidated for flood risk or drainage.

Elmbridge's Local Plan flood and drainage policy: CS26, DM13 and DM5

Elmbridge assesses flood risk and drainage against a small set of adopted policies. Core Strategy Policy CS26 (Flooding) is the flagship, backed by Development Management Plan Policy DM13 (Riverside development and uses) and Policy DM5 (Pollution), which carries the sustainable drainage requirement. The Flood Risk Supplementary Planning Document, adopted on 3 May 2016, is the practical guide to what a flood risk assessment for an Elmbridge site must contain.

What does Core Strategy Policy CS26 (Flooding) require?

Policy CS26 is Elmbridge's adopted flood policy, carried in the 2011 Core Strategy. It requires development to reduce the overall and local risk of flooding, and states that development should not constrain the natural function of the floodplain, either by impeding flood flow or reducing storage capacity. It must not increase flood risk elsewhere, and any residual risk must be safely managed. In practice, meeting CS26 means a site-specific flood risk assessment that reads every source of flooding, applies the sequential approach and shows the scheme is safe for its lifetime.

Policy DM13 (Riverside development and uses)

DM13 governs development along Elmbridge's rivers, where so much of the borough's flood risk sits. It requires proposals in flood-risk areas to take account of Core Strategy Policy CS26 and national guidance, and to respect water movement, flood risk, climate change, biodiversity and navigation. For a site near the Thames, Mole, Ember or Rythe, DM13 is the policy that ties the riverside setting to the flood risk assessment the council expects to see.

Policy DM5 (Pollution) and sustainable drainage

DM5 carries the borough's sustainable drainage requirement. It expects Sustainable Drainage Systems to be incorporated wherever practical to reduce the discharge of surface water to the sewer network and to protect water bodies and groundwater. It is the local policy hook for the drainage strategy that sits alongside an FRA, and it points straight at Surrey County Council's SuDS standards for the technical detail.

The Draft Elmbridge Local Plan 2037 was withdrawn from examination on 26 February 2025, and a new Local Plan is being prepared over 2025–2028, so these adopted policies and national policy currently apply. You can read the council's own flood risk planning policy and guidance on its website. Because the Flood Risk SPD flags a cumulative-impact concern, even a modest extension or infill plot in the wrong part of Elmbridge can attract more scrutiny than its size suggests — which is exactly where site-specific evidence earns its place.

Drainage and SuDS requirements for an Elmbridge development

Drainage is half of what an Elmbridge application has to answer, and it is Surrey County Council — as Lead Local Flood Authority — that sets the standard. Surrey expects surface water to be managed down the SuDS hierarchy, with discharge normally held to the greenfield runoff rate, and its published standard conditions set out exactly what a drainage scheme must demonstrate. This is the technical detail behind Elmbridge Policy DM5, and it is where a scheme most often meets an objection if the numbers are not right.

• Infiltration first, tested properly. Soakaway design must follow BRE Digest 365 infiltration testing, with a 1-metre unsaturated zone above the seasonal high groundwater level and half-drain times confirmed.
• Design storms. Hydraulic calculations must show the scheme manages the 1-in-30 and 1-in-100 (plus climate change) storm events, with a 10% allowance for urban creep.
• Controlled discharge. Where infiltration is unfeasible, discharge and storage are sized to the maximum rate on the decision notice — normally the greenfield runoff rate for the site.
• Exceedance routing. An exceedance flow plan must show that rainfall beyond the design event does not increase flood risk on or off the site.
• Maintenance and verification. The scheme needs a maintenance regime and, before occupation, a verification report with as-built drawings confirming it was built as approved.

Surrey provides separate design guidance for major and non-major development and offers chargeable pre-application SuDS advice, which is often worth taking on a complex site. Our sustainable drainage strategy and discharge of drainage conditions services are built around exactly these standards, and our free drainage calculators, BRE 365 infiltration calculator and geology and ground conditions map help you sense-check whether infiltration is realistic before design begins. The 2025 national standards for SuDS add further weight to getting this right.

The Elmbridge Strategic Flood Risk Assessment (April 2024)

Elmbridge published an updated Level 1 Strategic Flood Risk Assessment in April 2024, prepared by AECOM and replacing the earlier 2019 version. It maps flood risk across the borough from every source — rivers, surface water, groundwater, sewers and reservoirs — and is a living document that is refreshed as the Environment Agency updates its own mapping. It also gives applicants guidance for preparing the site-specific assessments that support planning applications.

A Level 2 SFRA is expected to follow, looking in more detail at flood risk within each flood zone to support the exception test where development is steered towards higher-risk land. For an applicant, the Level 1 SFRA is the starting point, not the finish line: it tells you the strategic picture, but the council and the Environment Agency still expect a report tuned to your actual site, its levels and its flow paths. Our explainer on what a Strategic Flood Risk Assessment is covers how these documents are used, and the council's SFRA 2024 page hosts the report itself.

What Elmbridge's local validation checklist requires

Elmbridge validates applications against a local list as well as the national requirements, and that list was updated for applications submitted from January 2025 to add the Elmbridge Design Code and Biodiversity Net Gain. For flood risk, the council is clear that failing to provide a flood risk assessment — and, where needed, the sequential and exception tests — is likely to delay determination and can lead to refusal. Getting the validation documents right first time is the quickest route through.

• Flood risk assessment. Required for sites in an area at risk of flooding, or of one hectare or more, and expected to address every source of flooding relevant to the site.
• Sequential and exception tests. Where the location needs justifying, these must accompany the application; their absence is a common cause of delay.
• Surface water drainage details. A drainage strategy demonstrating the SuDS hierarchy has been followed, sufficient for Surrey County Council to advise the council as Lead Local Flood Authority.
• A validated submission moves faster. Incomplete flood or drainage information is one of the most frequent reasons an application stalls at the validation stage.

The council's validation requirements set out the full local list. If you want the national picture on which developments trigger a report, our guides to flood zones 1, 2 and 3 and the flood risk vulnerability classification explain how the thresholds work.

How national flood risk policy applies in Elmbridge

Elmbridge determines applications within the national framework: the National Planning Policy Framework and its Planning Practice Guidance drive the sequential, risk-based approach, the Environment Agency's Flood Map for Planning defines the flood zones, and the national SuDS standards shape drainage. The council's local policies — CS26, DM13 and DM5 — sit on top of that scaffold rather than replacing it, so an Elmbridge assessment reads national policy and local evidence together.

• The NPPF and PPG. Set the sequential and exception tests and the safe-for-lifetime, no-increase-elsewhere principle — see our NPPF flood risk explainer and NPPF guidance service.
• Flood zones and climate change. The Environment Agency's mapping and its climate change allowances set the design horizon, which for many Elmbridge Thames-side sites is the governing factor.
• Critical drainage and surface water. Surface water risk can require a report even in Flood Zone 1 — see critical drainage areas and planning.
• Drainage conditions. Approval often comes with pre-commencement drainage conditions to discharge — our note on drainage information for planning conditions explains the detailed stage.

What's changing: Elmbridge and the new East Surrey council

Local government in Surrey is being reorganised. From 1 April 2027, Elmbridge, Epsom & Ewell, Mole Valley, Reigate & Banstead and Tandridge will be replaced by a single East Surrey unitary council, with elections to the new authority in May 2026 and a shadow authority running until the handover. It is a real change to who administers planning — but it does not change the flood risk and drainage tests a development has to pass.

The national framework, the Environment Agency's role and Surrey's role as Lead Local Flood Authority all carry through the transition, and the adopted Elmbridge policies — CS26, DM13 and DM5 — remain the starting point until any new plan replaces them. For applicants, the practical message is continuity: a flood risk assessment or drainage strategy prepared to the current standards will still be the right report on the other side of reorganisation. We keep our reading of the local policy landscape current so a submission is not caught out by the change.

Flood risk across Elmbridge's area

Elmbridge floods from more than one direction. The non-tidal River Thames runs along its northern edge through Molesey, Walton and Weybridge; the Mole and the engineered Ember join it from the south, with the Rythe and the Bourne threading the lower ground; and surface water overwhelms drains on the London Clay after intense rain. The benchmark events are the 1968 storms, when parts of Molesey stood several feet deep, and the winter of 2013–14, when the Thames and Mole flooded homes across the borough.

That multi-source picture is why a desktop map rarely satisfies an officer here: a site can be dry on the Flood Map for Planning yet exposed to surface water or the residual risk behind a defence. For the full local flood-risk detail on individual towns, see our area pages for Walton-on-Thames, Weybridge, Molesey, Cobham, Esher, Thames Ditton and Oxshott, or the wider Surrey overview. You can screen any site yourself on our flood risk map and live river levels map. The delayed River Thames Scheme is a reminder not to rely on defences that are not yet built.

How Unda helps with an Elmbridge flood risk and drainage submission

Unda are experts in the flood risk and drainage regime that applies in Elmbridge. We have prepared flood risk assessments and drainage strategies across the borough and the wider Thames and Mole valleys, and we deal directly with the Environment Agency and with Surrey County Council as Lead Local Flood Authority, guiding architects, planning consultants, developers, landowners, homebuyers and businesses through to a decision. We know how CS26, DM13 and DM5 and Surrey's drainage standards are applied in practice, and we write reports that answer their questions before they are asked.

Flood risk and drainage services for every situation in Elmbridge's area

• Flood risk assessment for planning. The core report for an Elmbridge planning application — see flood risk assessment for planning.
• Drainage and SuDS. A surface water drainage strategy, preliminary drainage design and groundwater assessment to Surrey's standards.
• Property purchase and mortgage. A homebuyer flood risk survey when a search or lender raises a concern.
• Insurance. An insurance flood risk assessment where cover has been restricted or premiums have risen.
• BREEAM. A BREEAM flood risk assessment structured around the Pol 03 credits.
• Sequential and exception tests. The locational sequential and exception tests where a site needs justifying.

Flood risk and drainage for an Elmbridge application: common questions